diff --git a/samples/nsudoku.pl b/samples/nsudoku.pl
index 44c4c88b1..83e033086 100644
--- a/samples/nsudoku.pl
+++ b/samples/nsudoku.pl
@@ -1,4 +1,5 @@
 :- use_module(library(clpz)).
+:- use_module(library(format)).
 
 sudoku(Rows) :-
         length(Rows, 9), maplist(same_length(Rows), Rows),